Method for producing wrought zirconium alloys

ABSTRACT

1. The method of producing zirconium-base alloys having improved mechanical properties and corrosion resistance, the steps comprising heating the alloy to a temperature within the all beta phase region for a period of time to place in solution the alloying components and impurities while preventing excessive grain growth, quenching the alloy at a rate of at least 90* F. per minute to a temperature below the all alpha phase temperature, reheating the alloy to a temperature within the all alpha phase region, hot working the alloy while in the alpha phase to effect a minimum reduction in cross-sectional area of at least 40 percent to final size, annealing the alloy at a temperature within the alpha phase, and thereafter cooling the alloy to room temperature.

United States Patent 1151 3,645,800

Mock et al. Feb. 29, 1972 [54] METHOD FOR PRODUCING WROUGHT 3,287,111 11/1966 Klepfer ..75/177 ZIRCONIUM ALLOYS [72] Inventors: James W. Mock, Franklin Township, Ex-

port; William C. Bowen, McKeesport, both of Pa.

[73] Assignee: Westinghouse Electric Corporation, Pittsburgh, Pa.

[22] Filed: Dec. 17, 1965 [21] Appl. No.: 515,297

[52] U.S.Cl .J. ..l48/ll.5

[51] Int.Cl......

[58] Field of Search ..75/177; 148/115 [56] References Cited UNITED STATES PATENTS 2,924,518 2/1960 Raine et al ..75/177 3,097,094 7/1963 Rubenstein et al.. ..75/177 3,121,034 2/1964 Anderko et al ..75/177 TEMPERATURE F Primary Examiner-L. Dewayne Rutledge Assistant ExaminerWayland W. Stallard Attorney-F. Shapoe 'EXEMPLARY CLAIM l. The method of producing zirconium-base alloys having improved mechanical properties and corrosion resistance, the steps comprising heating the alloy to a temperature within the all beta phase region for a period of time to place in solution the alloyingcomponents and impurities while preventing excessive grain growth, quenching the alloy at a rate of at least 90 F. per minute to a temperature below the all alpha phase temperature, reheating the alloy to a temperature within the all alpha phase region, hot working the alloy while in the alpha phase to effect a minimum reduction in cross-sectional area of at least 40 percent to final size, annealing the alloy at a temperature within the alpha phase, and thereafter cooling the alloy to room temperature.

7 Claims, 2 Drawing Figures WEIGHT TIN METHOD FOR PRODUCING WROUGHT ZIRCONIUM ALLOYS This invention relates to a method for producing zirconium alloys and more particularly it pertains to a method for heat treating and hot working such alloys to obtain improved mechanical properties and corrosion resistance.

Commercially pure zirconium has two important drawbacks; namely, low strength and highly variable corrosion behavior for use in nuclear reactors. The low neutron capture cross section of zirconium, however, makes it an attractive material for nuclear reactors. For that reason, zirconium base alloys have been developed which display enhanced physical and mechanical properties as well as acceptable corrosion resistance when used in high-pressure water, steam or other types of nuclear reactors.

Zircaloy is a generic designation for zirconium base alloys that are useful in the nuclear industry due to their low neutron capture cross section, good mechanical properties, high heat resistance, and corrosion resistance. Because of their low neutron absorption, zirconium base alloys such as zircaloy are useful as structural materials and fuel element cladding. U.S. Pat. No. 2,772,964 discloses zircaloy. Zircaloy is available as zircaloy-2 and zircaloy-4, which have compositions of l to 2 percent tin, 0.07 to 0.24 percent iron, 0.05 to 0.15 percent chromium, 0.007 to 0.08 percent nickel, and the balance is zirconium. The nominal composition of zircaloy-4 is 1.5 percent tin, 0.21 percent iron, 0.10 percent chromium, less than 0.007 percent nickel, and the balance being zirconium with incidental impurities.

Although zircaloy is superior in most respects to commercially pure zirconium for reactor purposes, it is desirable to improve the mechanical properties and corrosion resistance of the alloys in order to enable operation of a reactor at higher temperatures and for longer periods of time between refuel- A prior method of working the alloy for use in a reactor consisted of heating a billet to about 1,850 F., forging and/or hot rolling to an intermediate size at temperatures down to 1,650 F., surface conditioning the alloy body at room temperature, reheating to l,650 F., final forging and/or rolling to final size at about 1,550" F., and annealing the end product at about l,550 F. It has been found that superior mechanical properties and corrosion resistance may be obtained by subjecting the alloy to a beta quench, forging and/or rolling in the alpha condition and a modified annealing procedure during working of the alloy from the ingot stage to the final product.

Accordingly, it is a primary object of this invention to provide an improved working and annealing process for zirconium alloys to improve the corrosion resistance thereof.

It is another object of this invention to provide a method for fabricating zirconium base alloys into members by an improved heat treatment and hot working procedure.

Other objects and advantages of the invention will become apparent hereinafter.

For a better understanding of the nature and objects of the invention, reference is made to the following detailed description and to the drawings, in which:

FlG. l is a phase diagram of a zirconium-tin binary system; and

FIG. 2 is a graph depicting the heat treating and hot working cycle according to this invention.

This invention is particularly directed to processes capable of developing a ductile, high-strength zirconium base alloy having improved corrosion resistance when subjected to elevated temperatures in a steam or water atmosphere such as in a nuclear reactor.

The description of the process of this invention is particularly directed to zircaloy, and in particular, zircaloy-2 and zircaloy-4 alloys containing about 1.4 percent tin, but is exemplary of application of the process to zirconium base alloys generally. Changes in the tin content merely shift the limits of the two phase field as well as the magnitude of the alpha and beta phases in which the heat treatment and hot working are performed. For an alloy containing about 1.4 percent tin the upper alpha formation temperature is 1,650 F. and the lower beta formation temperature is 1,630 F.

More particularly, the invention consists of a method for working and heat treating zirconium base alloys including the steps, which are graphically illustrated in FIG. 2, of (A) heating an ingot of the alloy to a temperature of l,778:t50 F. until the center of the ingot reaches this temperature, (B) forging the ingot down to billet (or bar or plate) size such as about 1.25 to 2 inches in one transverse dimension, (C) reheating the billet to a temperature of l,990: :50 F. for 2 to 4 hours or until the center of the billet reaches a minimum of l,940 so that it is in the beta condition, (D) quenching the billet in the beta condition in water at a rate of F. or more per minute to room temperature, (B) surface conditioning the billet at room temperature to remove oxides, (F) reheating the billet to 1,450i50 F. long enough for the center to reach at least l,400 F., (G) hot working by forging or rolling the billet to final size with a minimum of 40 percent reduction to a transverse dimension of about 0.75 inch, (H) annealing at 1,4505'50" F. for a time of from 15 minutes to 2 hours, and (1) air cooling to room temperature.

The following schedule was applied to zirconium base alloys both zircaloy-2 and zircaloy-4, which are to be fabricated to a finished size of 0.75 inch or larger. An ingot of the alloy of a diameter of about 16 inches has a length of from about 4 to 7 (feet which is sufficient to provide an ingot weighing from 2,000 to 3,500 pounds. The term ingot refers to any cast member to be subjected to working to reduce it to desired size and shape. In heating the ingots or billets of the alloy, surface temperatures are determined by optical pyrometers or thermocouples are employed, and when temperatures are indicated without qualification as to their location the surface temperatures are meant. Initially, the ingot is heated to a temperature of 1,7783'50" F. for a period long enough for the center to reach such temperature at which point it is held for a maximum of 30 minutes to obtain uniform temperature. The ingot is then initially forged at this temperature without being allowed to cool below about l,400 F., down to a billet or plate size of about 7 inches minimum thickness which size is dictated by the maximum billet size acceptable by a rolling mill for a subsequent rolling operation. The billet may then be cut to convenient lengths.

The purpose of the initial heating operation at about 1,778 F. is to permit alloying elements and impurities to go into solution in the zirconium in order to improve the subsequent hot working or forging operation. However, the ingot is held at this temperature for a sufficient time but not exceeding 30 minutes at temperature to prevent excessive grain growth. As mentioned previously, the amount of the hot working or forging following the initial heating is dictated in part by the size of the rolling mill in which the billet is to be subsequently rolled after the beta quench D. In addition, the amount of the initial hot work or forging is controlled by the amount of reduction after the beta quench. That is, after the beta quench, the billet must be reduced by hot working or forging by an amount greater than 40 percent in order to obtain the desired final properties. i

If during the initial hot work or forging procedure from about l,778 F., the billet cools to a temperature below l,400 F. before the desired billet size is obtained, the billet is reheated (B one or more times to about l,500 F. and hot working or forging (B is continued until the desired billet size is obtained.

The primary function of the hot working or forging is to reduce the size of the ingot. During the operation, the grains are broken up. Due to the temperature drop, the alloying elements and impurities come out of solution at the grain boundaries.

After the initial steps A (heating to about 1,778" F.) and B hot working or forging) the resulting billet is reheated (C) into the beta phase until the entire billet is at a temperature of l,990* -J0 F. The billet is soaked at temperature for a sufficient time for the center of thebillet to reach a minimum of l,940 F. or for from 2 to 4 hours for the purpose of dissolving all alloying elements and impurities to produce a solid solution thereof. If the ingot is held at temperature for periods substantially longer than about 4 hours, there is excessive grain growth.

Thereafter, the billet is quenched (D) in water at a quenching rate of at least 90 F. per minute to a temperature below the alpha and alpha plus beta transformation; temperatures in order to retain the alloying elements and impurities in solution and to minimize grain growth. For that purpose, the water is preferably at room temperature and the minimum ratio of water to metal ranges from 15:1 to 25:1. Although the positions of the alpha and alpha plus beta transformation temperature varies with the amount of alloying elements, as shown in FIG. 1 for a 1.5 weight percent in tin composition, the alpha beta upper limit is approximately 1,650 F. and the corresponding alpha temperature is approximately 1,615 F. For other compositions appropriate temperatures for these regions are evident from FIG. 1. 1

After quenching to room temperature it is necessary to surface condition each billet, as by scarfing, to remove defects such as rolled-in oxides (ZrO to obtain optimum corrosion resistance. Such oxides develop when heated in ordinary furnace atmospheres, but may be avoided when controlled reducing atmospheres are used. Under the latter heating conditions, however, surface oxides are formed when the billet is quenched in water. If the alloy is to be used under circumstances where surface perfection and corrosion resistance is not of paramount importance, the alloy billet may be quenched down to a temperature of about 1,450 F. and then hot worked to final size as by step G.

The billet is then reheated (F) to a temperature of 1,450i5 0 F. for a time long enough for the center of the billet to reach a minimum of 1,400 F. and preferably about l,450 F. At those temperatures, the billet is within the alpha phase where good metal working properties are obtained without cracking during subsequent hot rolling or forging. During this stage there is a minimum of precipitation of the alloying elements and impurities at these temperatures. Another advantage of heating into the alpha-phase without going into the upper beta range is to obtain satisfactory metal working properties without growth into excessive grain sizes which develop at the higher alpha plus beta and beta-phase temperatures.

Thereafter, the billet forged and/or hot rolled to the final size of about 0.75 inch, although the technique applies to smaller and greater thicknesses as well. During this hot workingor forging step G, there must be a minimum of 40 percent reduction in size in order to break up the larger grain structures and to impart desirable physical properties including improved elongation. Although the prior heating step F was performed within the alpha-phase, the initial heating of the ingot at step A occurred in the beta-phase for which reason prior beta structure or large grains may have developed and persisted during the subsequent hot working step B and the quenching step D. For that reason, a reduction of at least 40 percent in size of the billet is necessary to achieve improved elongation by breaking up the larger grains. A typical grain size is ASTM 2 to and a preferred grain size of 5 to 8.

if during the hot working step G, the working temperature drops below 1,150 F., the billet is reheated (G1) one or more times to about l,450 F. for additional working or forging After completion of the alpha heating step F and the forging or hot rolling step G the billet is annealed (H) at 1,450i50 F. for to 30 minutes or for a sufiicient time to relieve stresses induced during the prior forging or rolling operation of step G. The annealing also provides for an equiaxed small grain structure which is conductive to high corrosion resistance.

The alloy is then air cooled (l) to room temperature and the alloy member is ready for use as by machining, cold working, fabrication, welding or the like.

Tensile properties in pounds per square inch (p.s.i.) at 600 F. are shown for alloys which have been beta quenched and alpha rolled and for alloys which have been beta quenched and not alpha rolled in the Table l as follows:

TABLE I Tensile Properties (p.s.i.) at 600 F.

The values in the table clearly indicate that by alpha rolling the mechanical properties of the alloy after being beta quenched are improved. The elongation has a definite increase of about 25 percent, and the yield strength and ultimate strength likewise show improvements in the amounts of 9 percent and 14.5 percent, respectively.

Clearly, this process provides an alloy material having improved ductility and tensile properties.

In addition, the beta quench and alpha hot working technique of this invention provide greatly improved corrosion resistance properties. Test results for the corrosion rate of zircaloy test coupons prepared by the beta quench and alpha hot working technique are shown in table ll. Comparison of the quenched and rolled zircaloy is made with published standard resultsfor corrosion tests.

TABLE ll Weight Gains of Test Coupons in 750 Steam at 1,500 p.s.i.

The smaller the weight gain for a given test the more satisfactory the test results. Weight gains are measured in milligrams per square decimeter (mg./dm. The maximum weight gain under this test tolerated for zircaloy used in a nuclear reactor is 38 mg./dm. in addition, by visual appearance, the zircaloy surface must have a continuous, black, adherent corrosion film with no corrosion film deflects. Thus, the zircaloy as beta quenched and alpha rolled meets the requirements of weight gain and visual appearance for use in a nuclear reactor.

The foregoing proceedings of beta quenching and alpha rolling and/or forging is applicable for zirconium alloys such as zircaloy-2 and zircaloy-4 having finished sizes of about three-quarters inch. Where, however, the alloy is used in larger cross-sectional dimensions, the material may be beta quenched and subsequently annealed in accordance with the indicated procedure and without the intermediate step of alpha rolling and/or forging.

It is understood that the above specification and drawing is exemplary and not in limitation of the invention.

What is claimed is:

l. The method off producing zirconium-base alloys having improved mechanical properties and corrosion resistance, the steps comprising heating the alloy to a temperature within the all beta phase region for a period of time to place in solution the alloying components and impurities while preventing excessive grain growth, quenching the alloy at a rate of at least 90 F. per minute to a temperature below the all alpha phase temperature, reheating the alloy to a temperature within the all alpha phase region, hot working the alloy while in the alpha phase to effect a minimum reduction in cross sectional area of at least 40 percent to final size, annealing the alloy at a temperature within the alpha phase, and thereafter cooling the alloy to room temperature.

2. The method of claim 1 in which the alloys have a composition consisting essentially of, by weight, from 1.0 percent to 2.0 percent tin, from 0.05 percent to 0.25 percent iron, from 0.05 percent to 0.15 percent chromium, from 0.007 percent to 0.08 percent nickel, and the balance essentially zirconium with incidental impurities, and the alloy is heated to a temperature ranging from l,950 to 2,050 F., quenching the alloy to a temperature below the all alpha phase at a minimum quenching rate of 90 F. per minute, hot working the alloy to effect a minimum reduction in the cross-sectional area of 40 percent while at a temperature within the range between l.l50 F. and l,500 F. annealing the alloy within the temperature range of l,400 to l,500 F., and cooling the alloy to room temperature.

3. The method of claim 2 in which the alloy is preliminarily heated to a temperature within the range from 1,725 to l,825 F., and hot worked at a temperature in excess of about 1,400 R, such hot working effecting a reduction in cross-sectional area to an amount in excess of a 40 percent reduction to final size said preliminary heating and hot working occurring of 1,950 to 2,050 F. 1

4. The method of claim 1 in which the alloy is preliminarily heated into the beta phase temperature range and preliminarily hot worked until the temperature reaches aminimum of 1,400 F., reheated to a temperature within the range between l,940 F. and 2,090 F. and thereafter quenched in water.

5. The method of claim 1 in which the hot working of the alloy to effect a minimum reduction of 40 percent in crosssectional area is performed above the temperature of l,l50 F., and in which the alloy member is air cooled from the annealing range of l,400 to l,500 F.

6. The method for producing alloys having a composition consisting essentially of, by weight, from 1.0 percent to 2.0 percent tin, from 0.05 percent to 0.25 percent iron, from 0.05 percent to 0.15 percent chromium, from 0.007 percent to 0.08 percent nickel, and the balance essentially zirconium with incidental impurities the steps comprising heating the alloy to a temperature ranging from l,725 to l,825 F., hot working the alloy above a temperature of about l,400 F., reheating the alloy to a temperature ranging from 1,950 to 2,050 F. for 2 to 4 hours, quenching the alloy to a maximum temperature below the alpha and the alpha plus beta range at a minimum quenching rate of F. per minute, hot working the alloy from atemperature of about 1,450 F. down to a minimum reduction of 40 percent in cross-sectional area to final size, annealing the alloy at a temperature within the all alpha phase region for one-quarter to 2 hours, and cooling the alloy to room temperature.

7. The method of claim 4 in which the ratio by weight of quenching water to metal is within the range between 15:1 and 

1. THE METHOD OF PRODUCING ZIRCONIUM-BASE ALLOYS HAVING IMPROVED MECHANICAL PROPERTIES AND CORROSION RESISTANCE, THE STEPS COMPRISING HEATING THE ALLOY TO A TEMPERATURE WITHIN THE ALL BETA PHASE REGION FOR A PERIOD OF TIME TO PLACE IN SOLUTION THE ALLOYING COMPONENTS AND IMPURITIES WHILE PREVENTING EXCESSIVE GRAIN GROWTH, QUENCHING THE ALLOY AT A RATE OF AT LEAST 90* F. PER MINUTE TO A TEMPERATURE BELOW THE ALL ALPHA PHASE TEMPERATURE, REHEATING THE ALLOY TO A TEMPERATURE WHILE IN THE ALPHA PHASE REGION, HOT WORKING THE ALLOY WHILE IN THE ALPHA PHASE TO EFFECT A MINIMUM REDUCTION IN CROSS-SECTIONAL AREA OF AT LEAST 40 PERCENT TO FINAL SIZE, ANNEALING THE ALLOY AT A TEMPERATURE WITHIN THE ALPHA PHASE, AND THEREAFTER COOLING THE ALLOY TO ROOM TEMPERATURE.
 2. The method of claim 1 in which the alloys have a composition consisting essentially of, by weight, from 1.0 percent to 2.0 percent tin, from 0.05 percent to 0.25 percent iron, from 0.05 percent to 0.15 percent chromium, from 0.007 percent to 0.08 percent nickel, and the balance essentially zirconium with incidental impurities, and the alloy is heated to a temperature ranging from 1,950* to 2,050* F., quenching the alloy to a temperature below the all alpha phase at a minimum quenching rate of 90* F. per minute, hot working the alloy to effect a minimum reduction in the cross-sectional area of 40 percent while at a temperature within the range between 1.150* F. and 1,500* F. annealing the alloy within the temperature range of 1,400* to 1, 500* F., and cooling the alloy to room temperature.
 3. The method of claim 2 in wHich the alloy is preliminarily heated to a temperature within the range from 1,725* to 1,825* F., and hot worked at a temperature in excess of about 1,400* F., such hot working effecting a reduction in cross-sectional area to an amount in excess of a 40 percent reduction to final size said preliminary heating and hot working occurring prior to the step of heating the alloy in the temperature range of 1,950* to 2, 050* F.
 4. The method of claim 1 in which the alloy is preliminarily heated into the beta phase temperature range and preliminarily hot worked until the temperature reaches a minimum of 1,400* F., reheated to a temperature within the range between 1,940* F. and 2,090* F. and thereafter quenched in water.
 5. The method of claim 1 in which the hot working of the alloy to effect a minimum reduction of 40 percent in cross-sectional area is performed above the temperature of 1,150* F., and in which the alloy member is air cooled from the annealing range of 1,400* to 1,500* F.
 6. The method for producing alloys having a composition consisting essentially of, by weight, from 1.0 percent to 2.0 percent tin, from 0.05 percent to 0.25 percent iron, from 0.05 percent to 0.15 percent chromium, from 0.007 percent to 0.08 percent nickel, and the balance essentially zirconium with incidental impurities the steps comprising heating the alloy to a temperature ranging from 1,725* to 1,825* F., hot working the alloy above a temperature of about 1,400* F., reheating the alloy to a temperature ranging from 1,950* to 2,050* F. for 2 to 4 hours, quenching the alloy to a maximum temperature below the alpha and the alpha plus beta range at a minimum quenching rate of 90* F. per minute, hot working the alloy from a temperature of about 1,450* F. down to a minimum reduction of 40 percent in cross-sectional area to final size, annealing the alloy at a temperature within the all alpha phase region for one-quarter to 2 hours, and cooling the alloy to room temperature.
 7. The method of claim 4 in which the ratio by weight of quenching water to metal is within the range between 15:1 and 25: 